1. Field of the Art
The present invention relates to control of electrical actuators, and more particularly to the control of limited angle electric torque motors.
2. Discussion of the Related Art
Limited angle torque motors are known and used for a variety of applications, including engine speed control. In this application, a limited angle torque motor may be used, for example, to control the operation of a fuel valve. The motor may be coupled to the fuel valve so that controlled rotation of the motor shaft through a given angle translates a controlled movement to the fuel valve, between fully closed and fully open positions. As greater engine speed is desired, the angular position of the motor is controlled to further open the fuel valve, limited by a maximum speed, fully open valve position. Likewise, when a decrease in engine speed is desired, the motor is controlled to move the fuel valve toward its closed position.
In this and similar applications, precision control of the motor is required in order to achieve satisfactory operation of a controlled device (fuel valve in the foregoing example). A variety of motors and controllers are presently known to provide controlled operation of control devices in applications such as the above-described speed control application. However, various shortcomings are known to persist in the approaches presently known.
For example, mechanically operated torque motors are known to be used in applications for controlling bidirectional movement of a controlled device. In these motors, a mechanical return, such as a spring, is used to urge the motor shaft in one direction toward a fixed or limit position, while electrically operative motor torque is used to actively drive the motor shaft in the opposite direction toward a second fixed or limit position. By way of illustration, a spring may be disposed to motivate a fuel valve to its closed position, while the motor actively drives the valve toward its fully open position. When the mechanical biasing force exceeds the torque applied by the motor, then the controlled device moves toward its closed position. Similarly, when the torque applied by the motor exceeds the mechanical biasing force, then the controlled device is motivated toward its fully open position. The controlled device may be maintained in an intermediate position by balancing the electrically operative torque applied by the motor with the mechanical biasing forcing.
Unfortunately, a number of shortcomings persist when such mechanically operated torque motors are used, particularly in precision control applications. One problem arises from the variable torque applied by the mechanical bias to the motor shaft, since the return force of a spring, for example, varies with the extension of the spring. Therefore, as the spring or other mechanical biasing mechanism extends throughout the range of motion defined by the limit positions of the controlled device, the force or opposing torque applied against the motor shaft is known to vary. Accordingly, the motor controller must be designed to compensate for the varying torque applied against the motor shaft as the motor is driven to effect movement of the controlled device, which complicates the design and increases the cost of the controller.
Another shortcoming of this approach results from the variable torque of the mechanical biasing mechanism over the life of the device. A spring, for example, after being stretched and relaxed a number of times, will realize a change in its elasticity, thereby resulting in a corresponding change in the torque that opposes the motor torque. As this occurs, the controlled application suffers a decrease in efficiency. To more particularly describe this phenomenon, consider that the controller is generally designed to account for various factors such as the inertia of the motor, as well as the opposing force applied by the mechanical biasing mechanism. As this opposing force changes over time, a corresponding degradation in performance of the controller is realized.
A related shortcoming is noted in the variation in force applied by the mechanical biasing mechanism at various operating temperatures. That is, the elasticity, and therefore the torque, of the mechanical biasing mechanism is affected by temperature changes. While some temperature compensation can be designed into the controller, such an approach unduly complicates the controller design and elevates its cost.
Another shortcoming in mechanically biased motors is noted in the excessive power consumption, and the related problems of heat dissipation. The motor current draw, and therefore power consumption, must be sufficient to offset the opposing force of the return spring, which is a substantially constant value, notwithstanding the elasticity variations noted above. This constant offset current not only increases the motor operational costs, but also gives rise to heat dissipation concerns.
A further shortcoming of a mechanically biased motor is noted in its transient response. Where the particular controlled application requires a fast response time, mechanically operated torque motors are not a favored choice. Except for the elasticity variations noted above, the torque applied by the mechanical biasing means against the motor shaft is substantially constant. Therefore, a large mechanically applied bias force will result in a fast response time in the mechanically biased direction, but will result in a slower response in the opposing, motor driven direction. Similarly, a small mechanically applied force increases the response time in the opposing, motor driven direction, but will correspondingly decrease the response time in the mechanically biased direction. Accordingly, the chosen biasing mechanism it typically selected by a compromise to realize moderate performance and efficiency in both directions.
Stepper motors provide an alternative motor design sometimes utilized in bi-directional motor applications. While stepper motors generally have a relatively simple digital or pulsed controller design, their performance is belied by the discontinuous rotation of the motor shaft in discrete increments, rather than a uniform rotation. Accordingly, stepper motors typically provide an unsatisfactory solution for applications that demand high-precision continuous movement of the controlled device. High resolution stepper motors are known, however, to provide a fine step resolution, but are very expensive and thus usually cost prohibitive. Additionally, the drive circuitry for stepper motors is generally more complex and therefore expensive to operate over a wide range of operating conditions--e.g., voltage, current, temperature, position resolution, etc.
A further shortcoming in the prior art is noted in analog control circuits directed to control mechanical torque motors. Specifically, analog control circuits are generally more complex and expensive than their digital counterparts. In addition analog controllers are more sensitive to temperature changes. Although they can be designed to operate effectively over a desired temperature range, such an approach complicates the design and elevates the controller cost.